Solid combustible propellant composition

ABSTRACT

A propellant composition is disclosed, comprising a chlorine-free solid oxidizer, a fuel, a polymer binder, and a dodecaborate salt.

BACKGROUND OF THE INVENTION

This invention relates to solid combustible propellant compositions for a variety of propellant applications.

Combustible solid propellants are well-known for a variety of applications, including but not limited to air bag inflators, inflator cartridges for portable pneumatic tools, rocket propulsion systems, as well as propellants for a variety of ballistic launch systems. Ammonium perchlorate has been widely used as an oxidizer in composite compositions that also include a high-energy fuel and a polymer binder. Ammonium perchlorate offers a number of desired performance features such as performance, processability, and burning rate. However, ammonium perchlorate causes environmental and health problems through the release of hydrogen chloride into the environment. The chronic exposure to perchlorates, even in low concentrations, has been shown to cause various thyroid problems. The problems from ammonium perchlorate in propellants can become acute in areas with localized persistent use of propellant compositions such as at rocket launch sites or munitions test ranges. Development of drinking water standards for perchlorate in the United States is discussed in Journal of Environmental Management, Volume 91, Issue 2, Pages 303-310 Katarzyna H. Kucharzyk, Ronald L. Crawford, Barbara Cosens, Thomas F. Hess.

In view of the above, there have been efforts to develop combustible solid propellant compositions that utilize oxidizers that do not contain chlorine. Ammonium nitrate has been proposed for use as an alternative oxidizer to ammonium perchlorate. However, the use of ammonium nitrate in propellant applications has been subject to certain difficulties or limitations. For example, ammonium nitrate-containing propellant compositions have been subject to one or more of the following shortcomings: low burn rates, or burn rates exhibiting a high sensitivity to pressure, as well as to phase or other changes in crystalline structure such as may be associated with volumetric expansion such as may occur during temperature cycling over the normally expected or anticipated range of storage conditions. For example, storage conditions for warehoused components or munitions can vary widely in a range from −40° C. to about 110° C. Changes of form or structure of the ammonium nitrate crystalline structure may result in physical degradation of the solid structure or composite of the propellant composition. In particular, ammonium nitrate is known to undergo temperature-dependent changes through five phase changes, i.e., from Phase I through Phase V, with an especially significant volume change of ammonium nitrate associated with the reversible Phase IV to Phase III transition. Furthermore, such changes, even when relatively minute, can strongly influence the physical properties of a corresponding combustible solid propellant and, in turn, adversely affect the burn rate of the combustible solid propellant.

It has been found that the phase change-induced degradation of cast, extruded or pelletized ammonium nitrate-containing compositions can be mitigated if the humidity is kept extremely low. However, maintaining such low humidity level is often impractical for most manufacturing situations, so various forms of phase-stabilized ammonium nitrate compositions have been developed. In particular, ammonium nitrate has typically been phase-stabilized by admixture and/or reaction with minor amounts of additional chemical species. For example, U.S. Pat. No. 5,071,630 teaches stabilization with zinc oxide (ZnO), U.S. Pat. No. 5,641,938 teaches stabilization with potassium nitrate (KNO₃), and U.S. Pat. No. 5,063,036 teaches stabilization with cupric oxide (CuO). U.S. Pat. No. 6,059,906 teaches stabilization with a molecular sieve age stabilizing agent and a strengthening agent. However, many prior art compositions utilizing alternative oxidizers to ammonium perchlorate suffer from poor burn rate or from a less than optimal combination of various factors such as density, caloric output, specific impulse, and volumetric impulse.

BRIEF DESCRIPTION OF THE INVENTION

According to an aspect of the invention, a propellant composition comprises:

a chlorine-free solid oxidizer;

a fuel;

a polymer binder; and

a dodecaborate salt.

According to another aspect of the invention, a method of discharging a propellant comprises:

combusting, in an enclosed vessel comprising at least one discharge opening, a composition comprising a chlorine-free solid oxidizer, a fuel, a polymer binder and plasticizer, and a dodecaborate salt as combustion catalyst; and

discharging combustion gases produced by combustion of the composition as a propellant through the discharge opening.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying figures, in which:

FIG. 1 is a schematic depiction of a propellant discharge device; and

FIG. 2 is a plot of burn rate of propellant compositions disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

Various chlorine-free oxidizers can be used in the invention, alone or in combination. Exemplary oxidizers include but are not limited to oxygen rich nitrates, periodates, iodates, metal oxides, or dinitramides. Nitrate, iodate, periodates, and dinitramide salts typically utilize ammonium, alkylammonium, or a metal as cation. Metal cations can include an alkali metal (e.g., potassium), an alkaline earth metal (e.g., strontium), transition or a post-transition metal (e.g., copper or bismuth). Tungsten, zinc, silver, and other non-toxic and environmentally friendly materials can be used as cations. Exemplary useful include cations, salts, and oxidizers that provide densities greater than ammonium nitrate which is 1.95 grams/cm³. Other exemplary useful oxidizers are those with a positive oxygen balance (O.B.) (e.g., potassium periodate with an O.B.=27.8). Useful pairings of cations and anions include potassium-periodate, bismuth-oxide, cupric-oxide, cupric-nitrate, bismuth-nitrate, lithium-periodate, and ammonium-periodate. Metal oxide oxidizers include oxides of bismuth, copper, tungsten, zinc, molybdenum, and various high density metals. In some embodiments, the metal oxide is capable of being reduced by a metal fuel in the propellant composition. The metal oxides decompose at combustion temperatures to produce oxygen that oxidizes the fuels present in the composition. Specific examples of oxidizers include ammonium nitrate, potassium periodate, potassium nitrate, strontium nitrate, bismuth oxide, and potassium dinitramide. Oxidizers can be present in the propellant composition at levels of about 30 wt. % to about 75 wt. %, more specifically from about 40 wt. % to about 68 wt. %, and even more specifically from about 50 wt. % to about 62 wt. %. Unless otherwise stated, all weight percentages disclosed herein are based on the total weight of the propellant composition.

In some embodiments, the oxidizer comprises a nitrate salt such as phase stabilized ammonium nitrate (PSAN). As mentioned above, ammonium nitrate can be phase stabilized such as in a co-crystal form with potassium nitrate or other salts, as disclosed by U.S. Pat. Nos. 5,071,630; 5,641,938; 5,063,036; and 6,059,906, the disclosures of each of which are incorporated herein by reference. Higher levels of PSAN can help reduce smoke emissions from combustion of the propellant while providing a low environmental impact. In some exemplary embodiments, the propellant composition comprises at least 33 wt. % PSAN. In some exemplary embodiments the propellant composition comprises at least 32 wt. % PSAN, more specifically at least 41% wt. %, and even more specifically at least 63 wt. %. Upper limits on the amount of PSAN will be dictated by performance specifications such as burn rate. In some embodiments, the propellant composition comprises less than or equal to 20 wt. % PSAN. In some embodiments, the propellant composition comprises less than or equal to 30 wt. % PSAN, more specifically less than or equal to 41 wt. %, and even more specifically less than 63 wt. %. The disclosed upper and lower PSAN content limits serve also to disclose a number of ranges of values for PSAN content in the propellant composition.

The fuel in the propellant composition can be provided by a variety of components. The polymer binder is of course a fuel source, and is discussed in further detail below. Additional fuel components can be included in the form of nitroplasticizers, nitraamines, or energetic additives. Typical nitroplasticizers include, but are not limited to nitrate esters, many liquid phase, such as trimethylol ethane trinitrate (TMETN), triethylene glycol dinitrate (TEGDN), triethylene glycol trinitrate (TEGTN), butanetriol trinitrate (BTTN), diethyleneglycol dinitrate (DEGDN), ethyleneglycol dinitrate (EGDN), nitroglycerine (NG), diethylene glycerin trinitrate (DEGTN), dinitroglycerine (DNG), nitrobenzene (NB), N-butyl-2-nitratoethylnitramine (BNEN), methyl-2-nitratoethylnitramine (MNEN), ethyl-2-nitratoethylnitramine (ENEN) or mixtures thereof. In an embodiment, the nitroplasticizer can be a 50-50 by weight mixture of TMETN and TEGDN. The nitroplasticizer can be present in the composition at levels up to about 40 wt. %, more specifically from 4-22 wt. %. Energetic additives are solid phase nitrogen-rich auxiliary fuel components. Examples of energetic additives include, but are not limited to, azodicarbamide (AZT), dinitroxydiethylnitramine (DNDEN), cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), 1-nitro-1,2,4-triazol-5-one (NTO), 1,1-diamino-2,2-dinitroethylene (FOX-7) or mixtures thereof. Energetic additives can be present in the propellant composition amounts up to about 40%.

In some embodiments, the fuel includes one or metal powders. As used herein, the term “metal powder” includes powders of metals and of metal hydrides. Examples of metal powders include but are not limited to aluminum, magnesium, zirconium, zirconium hydride, titanium, titanium hydride, aluminum-silicon alloy, magnesium-aluminum alloy, and boron or mixtures/alloys thereof. Particle sizes of the metal powders can range from about 10 nanometers to about 20 μm to, and more specifically from about 2 μm to about 10 μm. The amounts and particle sizes of metal fuel can vary depending on system design parameters. Generally, larger amounts of metal fuel increase combustion temperature and volumetric impulse, but in too large of an amount they can cause metal oxide precipitate in the propellant exhaust, which can reduce velocity and lead to equipment fouling and breakdown. In some embodiments, the propellant composition comprises from about 1 to about 25 wt. % of metal powder, more specifically from about 16 wt. % to about 22 wt. %. In embodiments where a reduced smoke propellant is needed, the propellant composition can comprise from about 1 wt. % to about 5 wt. % metal powder.

The dodecaborate salt is a salt of dodecahydrodecaboric acid such as cesium dodecaborate, potassium dodecaborate, sodium dodecaborate, lithium dodecaborate, ammonium dodecaborate, or tetralkylammonium dodecaborate. The salts can be characterized by the formula M⁺²[B₁₂H₁₂]⁻² where M is a metal or ammonium in a stoichiometric amount to balance the −2 charge of the dodecaborate anion. Dodecaborate salts are available from commercial chemical suppliers, and can be included in the propellant composition in amounts from about 0.1 wt. % to about 10.0 wt. %, more specifically from about 1.0 wt. % to about 5.0 wt. %, and even more specifically from about 2.0 wt. % to about 4.0 wt. %.

Other additives can be included as well, as known in the art, including but not limited to cure catalysts (e.g., triphenyl bismuth or butyl tin dilaurate, a metal acetylacetonate), nitrate ester stabilizers (e.g., N-methyl-4-nitroaniline (MNA), 2-nitrodiphenylamine, (NDA), ethyl centralite (EC), antioxidants (e.g., 2,2′-bis(4-methyl-6-t-butylphenol)) and amorphous carbon powder.

The polymer binder of the propellant composition can be a thermoplastic it can be a thermoset composition that relies on a chemical curing mechanism. The polymer binder can be present in the propellant composition in an amount ranging from about 5 wt. % to about 18 wt. %, and even more specifically from about 8 wt. % to about 12 wt. %. Thermoset polymer binder compositions can contain one or more resins having polyfunctional groups that react with other resin functional groups or with a polyfunctional curing agent having groups reactive with the resin functional groups. Examples of polyfunctional resins include hydroxyl-terminated polybutadiene (HTPB), hydroxy-terminated polyether (HTPE), polyglycol adipate (PGA), glycidylazide polymer (GAP), poly bis-3,3′-azidomethyl oxetane (BAMO), poly-3-nitratomethyl-3-methyl oxetane (PNMMO), polyethylene glycol (PEG), polypropylene glycol (PPG), cellulose acetate (CA) or mixtures thereof. Curing agents include, but are not limited to, hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), trimethylxylene diisocyanate (TMDI), dimeryl diisocyanate (DDI), diphenylmethane diisocyanate (MDI), naphthalene diisocyanate (NDI), dianisidine diisocyanate (DADI), phenylene diisocyanate (PDI), xylylene diisocyanate (MXDI), other diisocyanates, triisocyanates, higher isocyanates than the triisocyanates, polyfunctional isocyanates (e.g., Desmodur N 100), other polyfunctional isocyanates or mixtures thereof. In some embodiments, the curing agent has least two reactive isocyanate groups. If there are no binder ingredients with a functionality that is greater than 2, then the curative functionality (e.g., number of reactive isocyanate groups per molecule of isocyanate curing agent) must be greater than 2.0. If there are binder polymers with a functionality of two or less, then an isocyanate with functionality greater than two may be used. The amount of the curing agent is determined by the desired stoichiometry (i.e., stoichiometry between curable binder and curing agent). In some embodiments, the curing agent is present in the propellant composition in an amount of about 0.5 wt. % to about 5%.

The combustible solid propellant composition can be prepared by blending the above-described components, i.e., oxidizer, fuel, polymer binder (or components thereof, e.g., polyfunctional resin and polyfunctional curing agent), dodecaborate salt, and any additional or optional components in a mixing vessel. During the working time of the uncured resin composition, the mixture can be molded or cast into a desired shape or extruded and pelletized. After cure of the polymer binder is complete, the solid propellant can be fitted into a propellant module for use in various applications such as an airbag inflator or a rocket motor. An exemplary propellant module is depicted in the Figure, where propellant module 10 has a housing or vessel 12 with a solid propellant composition 14 therein. Upon activation of combustion by ignition device 16 (e.g., an electronic ignition device), combustion of the solid propellant composition 14 produces combustion gases 18 that are exhausted as propellant through opening 19.

The invention is further described in the following Examples set forth below.

Examples

Propellant compositions were prepared by blending the components specified in Table 1, molding the resulting mass into a shape for testing, and curing to form solid articles for testing. Density, volumetric impulse, specific impulse, and relative burning rate with 2% dodecaborate salt were measured, and the results are also set forth in Table 1. Burning rate for various mixtures of dodecaborate salts and oxidizers was measured for the test articles and the results are set forth in Table 2. Several of these tests are conducted according to methods described in MIL-STD-286. FIG. 2 shows the relative burning rates of propellant compositions with 2% a dodecaborate salt, a chlorine-free solids oxidizer, a fuel, and a polymeric binder.

TABLE 1 Example Formulations Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparison Propellant Ingredients Binder (Polyol + Isocyanate curative + 9-10% 9-10% 9-10% 9-10% 9-10%  9-10%  9.0-11.5% Stabilizers + Additives) Nitrate Ester Plasticizers 10-18%  5-10% 5-10% 5-10% 5-10% 10-18% 16-22% Metal powders fuels (example Aluminum) 1-17% 1-17% 2-15% 8-20    2-15%  8-18% 0% Azodicarbonamide (AZD) 0-12% 0-9%  0-9%  0-9%  0-9%  0-8% 15-20% Phase Stabilized Ammonium Nitrate (PSAN) 37-43%  40-49     38-48%  38-48%  52-64% 48-55% Potassium Periodate 8-20% — 3-16% — — Bismuth Trioxide — 8-20% 3-16% — — Strontium Nitrate — 8-30% — — Potassium Nitrate — 8-30% — — Burn rate catalysts Dodecahydrododecaborate salt (Alkali, 0.5-5.0%  0.5-5.0%  0.5-5.0%  0.5-5.0%  0.5-5.0%  0.5-5%   0% alkaline earth, ammonium, or alkylammonium dodecahydrododecaborates) Performance Properties Density (g/cm³) 1.7-1.95    1.7-1.98    1.7-1.9     1.7-1.8     1.7-2.1     1.60-1.70    1.55-1.59    Volumetric Impulse (lbf-s/in³) 14.3-15.5     14.5-16.0     14.5-15.5     14.1-15.6     14.4-15.8     13.0-15.7    11.2-11.5    Isp (sec) 205-235     200-240     210-240     220-246     202-235     235-255    195-205    Relative burning rate with 2% dodecaborae 0.20 4.0 2.6 2.1 2.1 salt at 1000 psi

TABLE 2 Catalyst to Oxidizer Ratio 10:90 20:80 30:70 Blends of Catalyst and Oxidizers Relative Linear Burning Cs₂B₁₂H₁₂ + KIO₄ 31 71 1000 Cs₂B₁₂H₁₂ + KNO₃ 17 704 Cs₂B₁₂H₁₂ + Bi₂O₃ 0.80 11 Cs₂B₁₂H₁₂ + Sr(NO₃)₂ 0.42 11 Cs₂B₁₂H₁₂ + PSAN 0.80 K₂B₁₂H₁₂ + KIO₄ 161 ((C₄H₉)₄N)₂B₁₂H₁₂ + KIO₄ 29 Cs₂B₁₂H₁₂ = Cesium Dodecahydrododecaborate; KIO₄ = Potassium Periodate; Bi₂O₃ = Bismuth Trioxide; KNO₃ = Potassium Nitrate; Sr(NO₃)₂ = Strontium Nitrate; CuO = Cupric Oxide; WO₂ Tungsten Trioxide; PSAN = Phase-Stabilized Ammonium Nitrate.

As shown in Table 1, much higher performance (density, impulse, volumetric impulse) is obtained with co-oxidizers and burning rate catalysts of this invention. Table 2 shows much higher burning rates when dodecaborate salts of are blended with co-oxidizers of the patent in ratios of 10:90, 20:80, and 30:70. Burning rates at ambient pressure combustion as high as 1000 in/sec with a ratio of 30:70. Table 2 also shows that higher burning rates are achieved as the level of the catalyst is increased for a ratio of 10:90 to 30:70.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A solid combustible propellant composition, comprising: a chlorine-free solid oxidizer; a fuel; a polymer binder; and a dodecaborate salt.
 2. The composition of claim 1, wherein the chlorine-free solid oxidizer compound comprises nitrate salt, a periodate salt, a metal oxide, or a nitramide salt.
 3. The composition of claim 2, wherein the chlorine-free solid oxidizer comprises ammonium nitrate, potassium periodate, potassium nitrate, strontium nitrate, potassium dinitramide, or bismuth oxide.
 4. The composition of claim 1, wherein the fuel comprises metal powder.
 5. The composition of claim 1, wherein the dodecaborate salt comprises a salt of a dodecaborate anion and a cation comprising an alkali metal, an alkaline earth, ammonium, or alkylammonium.
 6. The composition of claim 1, further comprising a nitroplasticizer or other energetic fuel.
 7. The composition of claim 6, comprising from 30-75 wt. % of chlorine-free oxidizer, 1-24 wt. % of metal powder, 5-18 wt. % of polymer binder, 5-22 wt. % of the nitrate ester plasticizer, and 0.1-10.0 wt. % of dodecaborate salt.
 8. The composition of claim 1, wherein the weight ratio of chlorine-free oxidizer to dodecaborate salt is from 4:1 to 60:1.
 9. A method of discharging a propellant, comprising: combusting, in an enclosed vessel comprising at least one discharge opening, a composition comprising a chlorine-free solid oxidizer, a fuel, a polymer binder and plasticizer, and a dodecaborate salt; and discharging combustion gases produced by combustion of the composition as a propellant through the discharge opening.
 10. The method of claim 9, wherein the chlorine-free solid oxidizer comprises nitrate salt, a periodate salt, or a metal oxide.
 11. The method of claim 10, wherein the chlorine-free solid oxidizer comprises ammonium nitrate, potassium periodate, potassium nitrate, strontium nitrate, or bismuth oxide.
 12. The method of claim 9, wherein the fuel comprises metal powder.
 13. The method of claim 9, wherein the dodecaborate salt comprises a salt of a dodecaborate anion and a cation comprising an alkali metal, an alkaline earth, ammonium, or alkylammonium.
 14. The method of claim 9, wherein the composition comprises from 30-75 wt. % of the chlorine-free oxidizer, 1-24 wt. % of the metal powder, 5-18 wt. % of the polymer binder, 4-22 wt. % of a nitrate ester plasticizer, and 0.1-10.0 wt. % of dodecaborate salt.
 15. The method of claim 9, wherein the weight ratio of chlorine-free oxidizer to dodecaborate salt in the composition is from 4:1 to 60:1 